This application relates to a linear power amplifier including a plurality of parallel connected solid state field effect transistors or the like for dividing and carrying of power current.
Various power circuits may include solid state amplifying devices for controlling of the load current through the load device or devices. Power amplifiers using field effect transistors of the metal oxide semiconductor base, identified by the acronym MOSFET, are recognized as highly satisfactory elements for both power switching and power amplification. MOSFETs are rugged and reliable devices, and have a high input impedance permitting simplification in the gate drive circuitry. The individual devices have limited current-carrying capability, and parallel circuits are often used whereby the devices share and thereby increase the load current rating. Equal sharing of the load current between the devices is significant to prevent one device carrying all or most of the load current with a possible rapid destruction of such device. A paper entitled "Paralleling Of Power MOSFETS In Linear Applications" was presented by Kim Gauen of Motorola, Inc., Semiconductor Product Sector, Phoenix, Arizona at the April 1984 Proceedings Of PCI. The Gauen paper discusses the use of the MOSFETs in a linear application and particularly the necessity of stabilizing the operating point of the individual MOSFETs. The problems are related to the variation in the characteristic of the individual devices and particularly the variation in the current carried at a give gate voltage of the devices. The solution set forth in the Gauen paper included addition of resistance in series with the source. The paper suggests the circuit stabilizes the operating point of MOSFET devices having widely different transconductance characteristics.
In certain applications, particularly very high voltage and low current applications, the inventor has discovered that the stabilizing system of the prior art has failed to produce a reliable power supply. For example, the energization circuit of a DC krypton arc lamp in a scientific laser power supply may include a high power variable ballast element functioning as an appropriate voltage controlled current sink. The laser requires a pulsed supply and the ballast element must establish a rapid linear response to an externally applied control voltage signal. The ballast element must have a voltage rating on the order of 400 volts, in order to allow a 350 volt direct current operation with an appropriate safety margin. An efficient packaging of the ballast element for appropriate dissipation of the thermal energy is essential. Using standard technology, the inventor discovered that a ballast element in the form of a plurality of paralleled MOSFETs resulted in almost immediate failure.
There is therefore a continuing need for a power MOSFET linear amplifier which operates at a high voltage and both high and low load currents, and wherein the drain current is a function of the gate to source voltage. In addition to the specific example of a pulsed laser source, some other typical applications for a multi MOSFET linear amplifier includes voltage controlled current source, voltage controlled resistor, ultrasonic amplifier, AF and RF amplification and electronic load bank.